Conventionally, a two-stroke engine has been used in a portable working machine. A two-stroke engine has a simpler structure than a four-stroke engine, and therefore can easily reduce its size and weight, and provide high output. However, a two-stroke engine consumes more fuel and discharges a larger amount of harmful substances than a four-stroke engine. Therefore, there is a demand to apply a four-stroke engine to a portable working machine taking into account influence on the environment (see, Patent Document 1).